1. Field of the Invention
The present invention relates to digital memory devices and, more specifically, to a non-volatile digital memory storage cell.
2. Description of the Prior Art
In digital systems, a non-volatile memory device is a device that is programmed to store a data value and then maintain that data value even after long periods in a powered-down state. Magnetic data recording disks are examples of non-volatile memory: data may be recorded on them and they will retain the data after long periods of storage without requiring any type of data refresh.
Certain types of solid state non-volatile memory devices store data in a permanent or near-permanent state. These types of non-volatile memory devices are often used to store the initial “boot-up” sequences of an operating system and programming code in single-application systems.
Many types of non-volatile memory devices are limited in data density as a result of the size of their constituent memory cells. One example of such memory cells employ e-fuses to set a permanent data state of each memory cell. To store a first value in the memory cell, the associated e-fuse is blown, to store a second value in the memory cell, the e-fuse is left intact. Such e-fuse based memory cells offer highly reliable permanent storage. However, they are bulky because the e-fuses are relatively large and they require large protection transistors. This limits the data density stored on an e-fused based memory device. Also, once an e-fuse is blown, then the bit stored on the memory cell is stored permanently and the device cannot be reprogrammed to store new data.
Negative bias temperature instability (NBTI) is a phenomenon that exists in p-type field effect transistors (PFET's) when they are subjected to heating for a given amount of time. NBTI is characterized by an increase in the threshold voltage and a decrease in drain current and transconductance in a PFET. Positive bias temperature instability (PBTI) is a similar phenomenon that exists in n-type field effect transistors (NFET's).
Therefore, there is a need for semi-permanent non-volatile memory that has a high data density.